FIG. 30 shows a conventional example 1 of a lighting apparatus disclosed in Document 1 (Japanese Patent Unexamined Laid-open Publication No. 9-73991). The conventional example 1 of the lighting apparatus comprises a DC power supply E, a lighting switch S, a DC-DC converter 1, a low-frequency inverter 2, an igniter 4, a discharge lamp La, and a control circuit 5. The control circuit 5 controls an output power so as to be a set value by detecting an output voltage and an output current of the DC-DC converter 1 as a lamp voltage and a lamp current of the discharge lamp La, calculating a lamp power from the detection results, and driving a switching element Q0 via a PWM drive unit 9 using an output power command value obtained based on the calculation result.
As shown in FIGS. 31A and 31B, when the discharge lamp La is actuated to be turned on, there is provided a period for inverting an output polarity of an inverter 2 (referred to as “electrode heating period”, hereinafter) at a frequency fd i.e., long inversion cycle) lower than an inversion frequency ft which is used at the time of a normal operation. The inversion frequency is set by an inversion frequency setting unit 10.
When the discharge lamp La is actuated to be turned on, if an OFF period before the discharge lamp is turned on is relatively long and an electrode temperature of the lamp is low, a lamp current detection value for the electrode heating period is great. Meanwhile, if the OFF period before the discharge lamp is turned on is short, since the electrode temperature at the time of start of lighting is higher as the OFF period is shorter, the lamp current detection value for the electrode heating period is small.
Thus, there is provided a control unit for varying an inversion cycle for the electrode heating period depending on the lamp current detection value. Thus, since the inversion cycle for the electrode heating period is increased as the lamp current detection value is smaller, the discharge lamp can be turned on in a state in which the electrode of the discharge lamp is sufficiently heated up. As a result, the discharge lamp is prevented from going off when the output polarity is inverted in the inverter circuit, so that the discharge lamp can be surely moved to a stable lighting state. In addition, according to the conventional example 1, the set value of the inversion cycle is varied only depending on the lamp current detection value or a lamp current accumulated value.
According to a conventional example 2 disclosed in Japanese Patent Unexamined Laid-open Publication No. 2002-216982), a discharge lamp is prevented from going off by adjusting an inversion cycle for an electrode heating period, without using a lamp current detection value or a lamp current accumulated value as shown in the conventional example 1. In this conventional example 2, an accumulated value of a lamp voltage detection value, an inverse number of the lamp voltage detection value, a lamp current command value and an accumulated value of the lamp current command value are used for adjusting an inversion cycle.
Since the lamp current just after the discharge lamp is turned on is greatly fluctuated, there may be a case that the current value is not accurately detected in the conventional example 1. Thus, as means for solving the above problem, there is suggested a control in the conventional example 2 as described below. That is, in order to provide a stable lighting operation, there is set an open loop control period in which a switching element for controlling a power of the DC-DC converter is driven at a given frequency or on duty. Then, a lamp voltage and a lamp current for the open loop control period are used as initial values of the lamp voltage and the lamp current detection values at the time of the start of the electrode heating period. Thus, an accurate and stable adjustment for the inversion cycle can be achieved for the electrode heating period.
In this conventional example 2, the set value of the electrode heating period is varied only depending on the accumulated value of the lamp voltage detection value, the inverse number of the lamp voltage detection value, the lamp current command value and the accumulated value of the lamp current command value.